The adaptive immune response is initiated by T cell-antigen presenting cell interactions in lymph nodes and the spleen during a 2-3 day period. The regulation of this interaction is likely to play a key role in the T cell proliferation and differentiation in normal immune responses and pathological processes such as autoimmunity. We have developed a hypothesis based on a T cell receptor (TCR) mediated stop signal and stable immunological synapse as the first stage in T cell activation. We have also discovered that this stop signal can be reversed by gradients of chemokines interacting with dominant chemokine receptors CCR7 and CXCR3, but not by chemokine gradients interacting with subordinate chemokine receptors CCR5 and CXCR4. We have a related hypothesis that dominant chemokines regulate sensitivity to T cells to antigen in vivo. We will test the stop signal hypothesis and the dominant chemokine hypothesis through a series of experiments. In Aim 1 we will determine the parts of a subordinate chemokine receptors that must be transferred to a dominant chemokine receptor to generate a chimeric receptor that binds a dominant chemokine, but interacts with the TCR signaling system as a subordinate chemokine receptor. In vivo experiments with knock in mice bearing subordinate versions of CCR7 and CXCR3 will allow us to test the hypothesis that these chemokines normally control polarity. In Aim 2 we will determine the basal, inflammatory and antigen stimulated migratory properties of naive T cells in mouse lymph nodes in vivo. This study will allow us to directly test the stop signal hypothesis in vivo. In the third aim we will use mice that are deficient in CCR7 ligands and the use of CC chemokine antagonists to test the role of chemokines in T cell sensitivity to antigen and in T cell migration in the lymph node. This aim will provide additional evidence regarding the stop signal and dominant chemokine hypotheses. The result of these studies will fill significant gaps in our understanding of how the microenvironment controls the immune response.